Technically, it should have been zoning restrictions keeping homes away from the plant. The town of West actually grew up around that plant - it essentially didn't exist until after the plant opened.

And maybe a little of what Texans like to call "common sense." Of course, you'd need to have enough education to know that fertilizer can explode. Having grown up not too far from West, I can see how the same thing could have happened in my home town, or might still happen in many other towns like West.